Messages

1871

Mach4 General Discussion / Re: Mach4 positioning error

« on: December 14, 2019, 04:45:04 PM »

Hi,

well i haven't got in to that much detail yet, most probably they left it like that to give the user space for starting up and tuning the motor/drive without causing any error.

This NO mere detail, it absolutely fundamental to the operation of any servo or closed loop device. With the
factory default settings you will get very poor results. The following error window is just a joke.

You may recall back in post #3 that I explained that the manufacturers deliberately set both parameters very wide
so that first time users can operate the servo without nuisance alarms but they were never intended to be in use
in a real CNC machine.

Do the calculations....don't just guess.

If parameter 11 (zero error window) is set to 4 counts:

4/10,000 x 25=0.01mm or 10um. This is the effective resolution of your X axis as a result of setting the zero error window to 4,
but your current setting (default) is 20 and therefore your effective resolution is 40um....way less impressive don't you think!

Likewise if you set the following error window to 20 counts then Mach will alarm and Estop if the mill lags the commanded toolpath
by 40um whereas the current (default) setting is 20,000 or 50mm. Would you expect an alarm or Estop before your machine
deviated from the toolpath by 50mm?

Craig

1872

Mach4 General Discussion / Re: Mach4 positioning error

« on: December 14, 2019, 01:18:55 PM »

Hi,
sorry misread the default setting for the following error window, factory default is 20,000 counts or two revolutions.

With a direct coupled 25mm pitch ballscrew that would mean that the X axis could be 50mm out of sync with the commanded
position WITHOUT an alarm!!!!

The factory default is for new users, serious users will shrink that WAY down, I have mine set to 20 counts.

Craig

1873

Mach4 General Discussion / Re: Mach4 positioning error

« on: December 14, 2019, 01:13:58 PM »

Hi,
parameter 11 is the 'zero error window' and parameter 12 is the 'following error window'

Different manufacturers use different names but they mean the same thing.

In this case parameter 11, the zero error window, is set to 20 counts as default. Thus if the actual (encoder)
position gets to within 20 counts of the commanded position the drive ceases to try to get any closer. Is this what
you want?. I think its too high, I set mine (8000 cpr) to 4 counts.

The reason for having a zero error window is to that the drive does not 'hunt'. If the drive attempts to reduce the error
to zero counts then it will overshoot by one count, and then reverse until it overshoots by one count in the other
direction, and keep repeating, called 'hunting' or 'dithering'.

Why don't you try setting parameter 11 to zero, and see what happens?

Parameter 12 is the following error window and its way WAY WAY too wide. Its factory default is 30000 counts,
that's three revolutions!!!. It should be something like 20 counts.

If the servos actual position lags the commanded position by a significant amount, called following error, it's supposed to alarm.
As it stands the servo can be miles and miles out of sync without an alarm, good for while you are experimenting with your
drive but absolutely bloody hopeless for CNC.

If you program parameter 12 to a very small value (<10) then the servo will fault frequently because the servo cannot
'stay within the following error window' and yet if the following error window is set too wide then the servo could lag
the commanded position by a large amount and cause bad inaccuracies in your work.

Craig

1874

Mach4 General Discussion / Re: Mach4 positioning error

« on: December 14, 2019, 06:20:24 AM »

Hi,
have you read the zero error window setting? With a 10000cpr encoder I would set it to 4.

Craig

1875

Mach4 Toolbox / Re: Writing the script for a canned cycle

« on: December 13, 2019, 02:14:15 PM »

Hi,
you could write the Gcode manually with the help of a spreadsheet but you don't need to.

Look at the Turning feature of the Lathe Cycles tab. click the ID turn button. Then you can set initial points, endpoints,
depth of cut, roughing cuts, spring passes and finish cuts. The cycle wizard will compose the Gcode.

Craig

1876

Mach4 General Discussion / Re: Mach4 positioning error

« on: December 12, 2019, 06:55:49 PM »

Hi,
what Russ is proposing is that open collector outputs from your BoB will result in 'missed' pulses by the drive, given that the drive is optimized
for differential inputs.

This would be detected by the same test as I proposed earlier.

If you command a move of 10mm and x number of steps were missed by virtue of open collector signaling, then surely a command of 20mm would
miss twice as many steps.

What I'm trying to establish is the error proportional to the distance moved or the number of moves.

P_actual=a x P_commanded + d

In an ideal world a would equal one and d would be zero. What we are trying to do is find the actual and existing inaccuracies described by the two
parameters a and d.

If Russ's proposal is correct then a will be close to but not exactly equal to one. If my contention is correct, ie zero window error, or I suppose some
type of backlash compensation error, then d will be non zero.

Craig

1877

Mach4 General Discussion / Re: Mach4 positioning error

« on: December 12, 2019, 03:16:14 PM »

Hi,

Dont get angry with me,

I didn't but you have not fully investigated either suggestion.

I understand that you are certain of the pitch and I would too under the same circumstances but then you are on the forum asking. That suggests
that you have tried all the things you can think of and still have no answer. Can you rule out a pitch error ABSOLUTELY?

If a servo has a zero error window set too high, say 100 encoder counts, it will approach its commanded position until it gets to within 100 counts and stop.
That is the meaning of the zero error window. If you MDI successive 1mm movement commands its possible that a wide zero error window, having a linear
equivalent of 4-5um, would occur at each movement with the inaccuracy accumulating at each move. If however you commanded one movement of 5mm
then with the same zero error window I would expect the same basic 5um inaccuracy in 5mm verses 25um inaccuracy had you commanded five successive
movements.

Is my logic wrong?

I can't fault your logic but the problem persists......

Your suggestion about noise is not impossible, just improbable. Is it likely that noise, by definition random electrical noise, would cause such a consistent
inaccuracy?. I doubt it, but neither do I have a simple test to prove or disprove that.

What I am suggesting is that you use such tests as you can devise to prove/disprove certain things. As items are eliminated off the list those remaining potential
causes must contain the answer.

Craig

1878

Mach4 General Discussion / Re: Mach4 positioning error

« on: December 12, 2019, 02:31:38 PM »

Hi,

I am sure about the pitches, these are metric and i ordered them directly from the manufacturer not from the distributor, so we can rule that out

Can you rule that out....have you actually measured it? It does seem unlikely given the source of supply but you are relying on someone elses say-so.

I think this also rules out the zero error window, any other suggestions?

How does this rule out the zero error window?. I would expect a fairly consistent error if indeed it is something like a zero error window fault.
I have no other suggestions, you have dismissed the two I have without investigation, would other suggestions meet a better fate?

Craig

1879

Mach4 General Discussion / Re: Mach4 positioning error

« on: December 12, 2019, 01:44:21 PM »

Hi,
another possible cause is the 'zero error window' being set to wide.

All servos, and to my knowledge, closed loop steppers have a programmable parameter called (by some manufacturers)
the 'zero error window'. What it means is that if the commanded position and the actual position, as measured by the encoder,
is within a (programmable) number of encoder pulses the drive will not try to close the error loop any closer.

It is common practice when the drive is shipped to set the zero error window and the following error window very wide so that
first time users experimenting with their new servo/drive are not plagued with following errors. In CNC practice those windows
need to shrink dramatically to have any chance of accurately following a toolpath.

If the zero window error is set to 100 encoder counts, say,  that could explain why your move apparently never completes
and therefore be inaccurate.

Craig

1880

Mach4 General Discussion / Re: Mach4 positioning error

« on: December 12, 2019, 01:23:07 PM »

Hi,
this post should be on the Mach4 General Discussion Board, I imagine tweakie will shift it.

If the error is consistent and identical in all axes then the steps per unit are wrong.

I'm guessing that the pitch of the screws is 1 inch (25.4mm) not 25mm.

You may have seen posts where users believed they had 0.2 inch pitch screws (5.08mm) when in fact they
had very common metric screw of 5mm pitch. Depending on the supplier they may not know or care about a detail
like that, they have given enough information for a sale...and that's all they care about.

Can you use Mach and the current steps per unit value to have the stepper rotate exactly one revolution? If you can do so
measure the distance traveled......and that is the actual pitch which may be at variance to what you believe.

Craig